In recent years, an axial flow fan of high power output and revolution has been required in the market. With such a fan, when the electric power supply of the fan is interrupted for the maintenance or the inspection of the device, including the fan itself, a long period of time passes before the propeller of the fan stops.
Accordingly, if a finger, or other foreign object, is inserted into the still rotating fan by mistake, injury or damage may result.
In order to avoid the above defect, a finger guard or shield may be mounted. However, this increases noise, cost and effort. Accordingly a fan having no such defects has been required.
FIG. 6 shows a drive circuit for a conventional electric fan, wherein a reference numeral 1 denotes a regulator circuit portion, 2 denotes a drive signal generating circuit portion, 3 and 4 denote NPN transistors, 5 to 12 denote resistors, 13 and 14 denote NPN digital transistors, 15 and 16 denote MOSFETs of an upper stage side, 17 and 18 denote MOSFETs of a lower stage side, 19 denotes an exciter coil of an electric motor for driving a rotor, 20 denotes a capacitor, reference letter A denotes a voltage of an electric power supply, B and C denote upper stage side drive signals, and D and E denote lower stage side drive signals.
The drive circuit shown in FIG. 6 is operated normally by switching MOSFETs 15 and 16 of the upper stage side and MOSFETs 17 and 18 of the lower stage side by drive signals B, C, D and E applied from drive signal generating circuit portion 2. Voltage A of the electric power supply is lowered rapidly as shown in FIG. 7 when the electric power supply is interrupted, so that MOSFET 15 of upper stage side is turned OFF because the gate voltage thereof is lost, and MOSFET 17 of lower stage side is turned OFF because the gate voltage D′ thereof is discharged through resistors 10 and 9. A long time passes before the rotation of the fan is stopped because the rotation of the fan is reduced only by the cogging torque, mechanical friction of the bearing, and the flow resistance of the fan propeller. Accordingly, during inspection and maintenance of the fan, even if the electric power supply of the fan is interrupted, the propeller may still be rotating, so that if a finger, or other foreign object, is inserted into the rotating fan by mistake injury or damage may result.
A drive circuit for an electric fan as shown in FIG. 8 is publicly known. The drive circuit of FIG. 8 utilizes a mechanical relay 22 having a B contact which is inserted between the terminals of the electric power supply so as to short the exciter coil of the motor 19 by the B contact.
In the braking device as shown in FIG. 8, the B contact of the relay 22 is opened when the voltage of the electric power supply is applied, and MOSFETs 15 and 16 of the upper stage side and MOSFETs 17 and 18 of the lower stage side are turned OFF when the electric power supply is interrupted. The B contact is closed when no voltage is applied on the input side of the mechanical relay 22, so that the exciter coil of motor 19 is shorted, such that an electromagnetic brake is generated and the propeller of the fan is stopped forcibly. In the above conventional braking devices, however, a relay of large contact capacity must be used, a large inside space for installing therein a large mechanical relay is required in the fan, and the reliability of the mechanical relay is low.